This invention relates to a method of manufacturing thermal insulation systems for bulk cargo tanks used in the transportation and/or storage of liquefied and/or compressed gases. More particularly, it is concerned with the manufacture of an improved system for inhibiting the fracture of and maintaining substantially uniform stress levels in various parts of one or more layers of cryogenic foam used to thermally insulate and isolate bulk cargo tanks from the hull structures of vessels in which they are mounted. This system is particularly applicable to the installation of insulation material in converging wall areas which define a corner section of a cryogenic liquid containment structure.
Many proposals have been made to date to alleviate and/or compensate for the severe static and/or dynamic stresses imposed on liquefied gas bulk cargo tanks and particularly the corner sections of shipboard tanks during use. Static stresses can be caused by the particular water ballast conditions of the cargo vessel, or by the extreme temperature variations that occur in such tanks during normal use as well as during loading and unloading. The severe dynamic stresses, set up in such shipboard tank structures, can be due, for example, to liquid cargo accelerations and the sloshing of the liquid cargo in the tanks during ocean transport as well as from the deflections and bendings of the transport vessel itself when moving through heavy seas.
Such proposals have resulted in the rather complex corner structures, etc. illustrated in prior art U.S. Pat. Nos. 3,150,794, 3,490,639, 3,319,431, 3,406,858, 3,622,030, 3,613,932, 3,687,087, 3,712,500, 3,757,982 and 3,780,900. These patents are generally representative of the several principal type cryogenic liquid containment systems used today in ocean-going transport vessels and to which the instant invention is applicable. The tanks of such systems are commonly referred to as "prismatic free-standing tanks," "spherical free-standing tanks," "semi-membrane tanks," and "membrane tanks" and reference may be made to a paper presented by William DuBarry Thomas et al to the Society of Naval Architects and Marine Engineers on Nov. 11-12, 1971, and entitled "LNG Carriers-- The Current State of the Art" for a detailed discussion of these different type tanks and their respective merits.
As indicated by the aforesaid patents, attempted solutions to the loading and stress problems have tended to concentrate on improving the metallic or other structural supports for the corner sections of a given tank in a cryogenic containment system rather than on improving the structure of the cryogenic insulation as such. Other proposals have emphasized compensating for only a single type of stress, e.g. static or dynamic, rather than being addressed to solving both types of stresses.
The above problems have been further aggravated in the case of acceptable cryogenic urethane foams, such as those discussed in U.S. Pat. No. 3,757,982 and copending application Ser. No. 378,138 of Herbert H. Borup, filed July 11, 1973 now U.S. Pat. No. 3,929,247. Such foams, when emplaced and fully cured, frequently become relatively rigid and brittle and will tend to crack undr severe stresses unless adequate compensation is made therefor, such as by use of the complex balsa wood and/or plywood supporting structures or pads of the prismatic free-standing tank system, etc.
The instant invention is concerned with a simplified procedure for compensating for and/or alleviating dynamic as well as static stresses imposed on the insulation particularly in the corner section of a cryogenic bulk cargo tank structure. This is accomplished by a thickness reduction of selected portions of the insulation itself whereby the insulation in such corner section will tend to act as a hinge and flex under loads rather than cracking while at the same time uniformly distributing loads to other flat or planar portions of the insulation. For the purposes of this specification and claims, a corner structure shall be considered as comprising at least two convergent walls.